Conventionally, the magneto-optical disk is known as a rewritable disk-shaped recording medium. Recently, various techniques have been developed for improving the recording density of the magneto-optical disk. One such technique has recently been disclosed in the Japanese Unexamined Application Publication No. 290496 of 1994, for example.
The playback technique disclosed in the above Japanese Unexamined Application Publication is a magnetic domain wall displacement detection type ultra-high resolution magneto-optical playback technique, called “DWDD (domain wall displacement detection)”, used on a magneto-optical disk having at least three magnetic layers: displacement layer, switching layer and memory layer and which exploits the fact that the magnetic domain recorded in the memory layer will substantially be extended in the displacement layer. DWDD is to detect a magnetic domain wall displaced in an area in the displacement layer that corresponds to an area where a magnetic coupling between the memory and displacement layers corresponding to an area in the switching layer, irradiated with a read laser light for reading information signal and having reached a temperature higher than the Curie temperature, is broken, whereby a magnetic domain recorded in the memory layer is substantially extended in size in the displacement layer to increase read carrier signal.
The DWDD-based magneto-optical playback method (will be referred to as “DWDD playback method” hereunder) will be described in detail herebelow. In FIG. 1A, a magneto-optical disk to which the DWDD playback method is applied is generally indicated with a reference 100. As shown, the magneto-optical disk 100 is constructed of three magnetic layers including a displacement layer 101, switching layer 102 and memory layer 103. In the magneto-optical disk 100, the memory layer 103 is magnetized in different directions of arrows M1 and M2 as shown in FIG. 1A because of forward and reverse spins to record data as long as a magnetic domain. In this magneto-optical disk 100, a boundary between two successive magnetic domains 104 magnetized in different directions is a domain wall 105. It should be noted that an arrow R in FIG. 1A indicates the rotating direction of the magneto-optical disk 100.
When a read laser light BM is incident upon the magneto-optical disk 100, a temperature is distributed there as shown in FIG. 1B since the laser light BM provides a local heating. It should be noted that a reference Ts indicates a Curie temperature of the switching layer 102 and an area Dp in the switching layer 102 where the temperature is higher than the Curie temperature Ts will be demagnetized. In the magneto-optical disk 100, the demagnetization in the switching layer 102 will lead to disappearance of the force of switched connection between the demagnetized area Dp in the switching layer 102 and the memory layer 103. Thus in the magneto-optical disk 100, the domain wall having a lower coercivity as an area in the displacement layer 101 that corresponds to the area Dp is singly displaced toward a higher-temperature side indicated with an arrow S in FIG. 1A. Such a displacement of the domain wall will take place each time the domain wall existing in the displacement layer 101 correspondingly to each recorded mark in the memory layer 103 reaches an isothermal curve of the temperature Ts as the magneto-optical disk 100 is scanned. With detection of such a domain wall displacement, data recorded in the memory layer 103 of the magneto-optical disk 100 is substantially extended and read. It should be noted that the domain wall is displaced to near a spot where the temperature is highest, for example, the rear of a beam spot.
The DWDD playback method is one of the important playback methods permitting to read a very large signal from a very small recording domain, as well, of which the period is smaller than the critical optical resolution of a read laser light, with a high density without having to change the wavelength of the read laser light, numerical aperture (NA) of objective lens, etc. Say, the DWDD playback method allows to considerably improve the recording density in the scanning direction of laser beam spot.
On the other hand, the recording methods includes mainly two types: optical modulation and magnetic field modulation. The magnetic field modulation permits to record a mark in smaller size and increase the volume of data to be recorded correspondingly.
In the magnetic field modulation type recording, a recording laser light whose output level is higher than at the time of disk playback is projected onto the magneto-optical disk 100, a modulation magnetic field corresponding to a signal to be recorded is applied to the magneto-optical disk 100 to write the signal to be recorded, as a mark, to a desired recording track. FIG. 2 shows the relation in shape between a beam spot SP defined by a write laser light on a recording track 100a and the mark 110 recorded to the memory layer 103 of the magneto-optical disk 100.
In the field modulation type recording, with the write laser light being projected onto the magneto-optical disk 1A, record is sequentially made by a modulated field to a hot spot 111 taking place nearly in the center of the beam spot SP correspondingly to the shape of the beam spot SP, with the result that the mark 110 will be recorded there, as shown in FIG. 22. Where the hot spot 111 takes place depends upon the shape of the beam spot SP. So, when the beam spot SP is generally circular as shown in FIG. 2, the recorded mark 110 will be shaped to copy the rear end of the hot spot 111. Namely, the mark 110 thus recorded will have a fletching-like shape. More specifically, the shape of an edge portion 110a, defining the profile, of the recorded mark 110 depends upon the curvature of the rear end of the beam spot SP. It should be noted that the temperature of the hot spot 111 will exceed a temperature higher than the Curie temperature in the memory layer 103 and it is different from a temperature of a hot spot taking place due to irradiation of a read laser light at the time of disk playback.
In the DWDD playback method, the mark 110 recorded by the field modulation type recording is reproduced by detecting a displacement of the edge portion 110a developed by heating with a projected read laser light. Thus, the DWDD playback method can improve the linear density of the magneto-optical disk.
In the production of the above-mentioned magneto-optical disk, a recording layer to which information is recorded, a protective layer for the recording layer, etc. are formed on a substrate. The layers just formed are magnetized in different directions, which will cause noises when information is written to or read from the disk. Once information is recorded to the disk, the noises to the recording track on the magneto-optical disk disappear. However, since recording is made only the recording track, so the disk will remain magnetized in different directions. In a disk having a small track pitch, the remaining nonuniform magnetization will cause a cross talk which degrades the signal S/N (signal-to-noise) ratio and hence the signal quality.
To solve this problem, the magneto-optical disk is initial-formatted in the final process of the production or just before shipment from the factory. Generally, the bulk erase method is used for initial formatting of mass-produced magneto-optical disks. In this method, the magneto-optical disks are placed in a furnace at a temperature higher than the Curie point of them to momentarily be heated to a high temperature so that the disk will be magnetized uniformly in the same direction. However, the ultra-high resolution magneto-optical disk such as DWDD type disk, even if processed by the bulk erase method, incurs much noise during information recording or reproduction and shows no good detrack characteristic at the time of disk playback.
Also, the biaxial actuator for focusing and tracking a light beam scanning a magneto-optical disk generally uses an electromagnetic force as the driving force and so it is provided with a magnetic circuit including a magnet. The magnet included in the magnetic circuit produces a leakage flux under the influence of which noises will be increased during recording or reproduction of information to or from the magneto-optical disk.